1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a laser printer, a multifunction apparatus, or a copier. More particularly, the present invention relates to a fixing device for fusing a toner image onto a recording medium, and an image forming apparatus having the same.
2. Description of the Related Art
In general, electrophotographic image forming apparatuses such as laser printers, multifunction apparatuses, or copiers, print out a desired image on a recording medium through a series of image formation processes including charging, laser exposure, developing, transferring, and fixing.
During the fixing process, a toner image transferred onto the recording medium is fixed by heat and pressure applied by a fixing device to permanently form the image. The fixing device typically comprises a fixing roller heated by a heating source, and a pressing roller for pressing the recording medium against the fixing roller.
FIG. 1 shows a conventional fixing device 1 in an electrophotographic image forming apparatus. The fixing device 1 includes a fixing roller 11 comprising a cylindrical metal core pipe 11a and a resilient layer 11b enclosing an outer surface of the core pipe 11a. The resilient layer 11b is coated with a non-cohesive layer 11c. 
As shown in FIG. 2, a heating source, such as a halogen lamp 12, is mounted inside the fixing roller 11. The halogen lamp 12 generates heat in the fixing roller 11. Therefore, the fixing roller 11 is heated by radiant heat from the halogen lamp 12.
The pressing roller 13 is mounted at a lower part of the fixing roller 11. The pressing roller 13 comprises a cylindrical metal core pipe 13a and a resilient layer 13b enclosing an outer surface of the core pipe 13a. The resilient layer 13b is coated with a non-cohesive layer 13c. 
The pressing roller 13 is resiliently supported by a spring device 17, thereby biasing a recording medium 14, such as paper, passing through a fixing nip between the fixing roller 11 and the pressing roller 13 with a predetermined pressure.
Accordingly, while passing through the nip between the fixing roller 11 and the pressing roller 13, a toner image 14a transferred on the recording medium 14 is heated and pressed. As a result, the toner image 14a is fused on the recording medium 14 by the heat and pressure applied by the fixing roller 11 and the pressing roller 13.
In the conventional fixing device 1, however, the fixing roller 11 and the pressing roller 13 form the fixing nip by contacting each other. Therefore, when the rotational speeds of the fixing roller 11 and the pressing roller 13 are increased for high-speed printing, the time that the recording medium 14 bearing the toner image 14a stays at the fixing nip between the fixing roller 11 and the pressing roller 13 decreases. Therefore, the toner image 14a and the recording medium 14 may not be applied with the heat sufficiently or evenly. Accordingly, the incompletely fixed toner image 14a may cause deterioration of image quality.
To overcome the reduced time of the recording medium 14 at the fixing nip when performing high-speed printing, in the conventional fixing device 1, the outer diameters of the fixing roller 11 and the pressing roller 13 or the thicknesses of the resilient layers 11b and 13b have been increased.
When increasing the outer diameters of the fixing roller 11 and the pressing roller 13, however, the volume of the image forming apparatus increases. Furthermore, as the size of the fixing roller 11 increases, the heat capacity and/or warm-up time for heating the fixing roller 11 should be increased. This produces an increase in manufacturing costs, and an increase in power consumption.
When increasing the thickness of the resilient layers 11b and 13b, on the other hand, the warm-up time for heating the fixing roller 11 needs to be increased. Also, the heating temperature for the fixing roller 11 should be increased to maintain the fixing temperature of the resilient layers 11b and 13b. Accordingly, power consumption increases. Furthermore, when increasing the heating temperature for the fixing roller 11, the lifespan of the fixing roller 11 and the pressing roller 13 may be shortened due to deterioration of the resilient layers 11b and 13b, and/or deterioration of the contacting portion between the resilient layers 11b and 13b and the metal core pipes 11a and 13a. 
In addition, to prevent deterioration of fixing performance, the pressure of the pressing roller 13 applied to the fixing roller 11 has been increased by strengthening the resilience of the spring device 17 in the conventional fixing device 1.
However, when the pressure of the pressing roller 13 is increased, the resilient layers 11b and 13b of the fixing roller 11 and the pressing roller 13 may be deformed or distorted. Therefore, the heat and pressure of the fixing roller 11 and the pressing roller 13 may be applied unevenly to the toner image 14a and the recording medium 14, thereby causing deterioration of image quality. Furthermore, when the recording medium 14 cannot move smoothly due to excessive pressure, paper jams frequently occur. In addition, because the driving torque for driving the fixing roller 11 is increased, a driving motor for driving the fixing roller 11 may be damaged by the increased load.
Consequently, there is a need for an improved fixing device, which is capable of guaranteeing stable image quality when performing high-speed printing without having to increasing the outer diameters of the fixing roller 11 and the pressing roller 13 or thickness of the resilient layers 11b and 13b or increasing the pressure of the pressing roller 13.